I thought for sure this game had a link on the TFP but I can't find anything. Anyway, here's the link:
http://officiallinerider.com/play.html
It's just a fine little game where you make your own track for the little guy to sled down. I suggest playing the game for at least a few minutes before clicking any other links.
Now, Let's check out some of badass stuff people have created!
Line Rider Die at the slopes
http://youtube.com/watch?v=2SVMFCZgvNM
Jagged Peak Adventure
http://youtube.com/watch?v=bcu8ZdJ2dQo
Urban Run
http://youtube.com/watch?v=sZ4S-MYoQKQ
Super Mario!
http://youtube.com/watch?v=rn8ULJGboOE
Make Believe!
http://www.youtube.com/watch?v=fTnT66LGrdY
And my favorite...
Discarded
http://youtube.com/watch?v=cW44BpXpjYw
The way he uses the same ramp like 5 times is so great.
More links on youtube just search "line rider"